Optical system for taking image

ABSTRACT

An optical system for taking image comprises three lens elements with refractive power, from the object side to the image side: a first positive lens element having a convex front surface and a concave rear surface, and the front surface being aspheric; a negative plastic second lens element having a concave front surface and a convex rear surface, and the front and rear surfaces thereof being aspheric; a positive plastic third lens element having a convex front surface and a concave rear surface, the front and rear surfaces thereof being aspheric; and an aperture stop located between the first and second lens elements for controlling brightness of the optical system. The focal length of the first lens element is f1, a focal length of the second lens element is f2, a focal length of the optical system is f, and they satisfy the relations: f/f1&gt;0.95, |f/f2|&gt;0.34.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical system, and moreparticularly to a miniaturized optical system for taking image used in acamera mobile phone.

2. Description of the Prior Art

In recent years, with the popularity of camera mobile phone, the opticalsystem for taking image has been becoming thinner and thinner, and thephotosensitive assembly of a general digital camera is nothing more thanCCD (Charge Coupled Device) or CMOS (Complementary Metal OxideSemiconductor). Due to the advancement of the semiconductor technique,the pixel size of photosensitive assembly has been being reducedcontinuously, and the development of the miniaturized optical system fortaking image is toward the high resolution field. Therefore, the demandfor the image quality is increasing gradually.

A conventional mobile phone's lens assembly usually consists of threelenses: from the object side to the image side: a first lens elementwith positive refractive power, a second lens element with negativerefractive power and a third lens element with positive refractivepower, thus forming the so-called type of Triplet. To correct theaberration, the optical system usually takes the form of “front aperturestop”. However, the arrangement of front aperture stop will increase theoccurrence of stray light and the sensitivity of the optical system.

The present invention has arisen to mitigate and/or obviate theafore-described disadvantages.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to improve the imagequality and effectively reduce the sensitivity of the optical system,the present invention provides a whole new three-lens type opticalsystem.

An optical system for taking image in accordance with the presentinvention comprises three lens elements with refractive power, from theobject side to the image side:

a first lens element with positive refractive power having a convexfront surface and a concave rear surface, the front surface of the firstlens being aspheric;

a plastic second lens element with negative refractive power having aconcave front surface and a convex rear surface, the front surface andthe rear surface of the second lens being aspheric; and

a plastic third lens element with positive refractive power having aconvex front surface and a concave rear surface, the front surface andthe rear surface of the third lens being aspheric;

wherein an aperture stop of the optical system is located between thefirst lens element and the second lens element for controllingbrightness of the optical system.

In the optical system for taking image, the refractive power of theoptical system for taking image of the present invention is mainlyprovided by the first lens element with positive refractive power, thesecond lens element with negative refractive power is generally used tocorrect the chromatic aberration, and the third lens element serves tobalance and correct the various aberrations caused by the system.

In the optical system for taking image, the front surface and the rearsurface of the first lens element with positive refractive power areconvex and concave, respectively. The front surface and the rear surfaceof the second lens element with negative refractive power are concaveand convex, respectively. And the front surface and the rear surface ofthe third lens element with positive refractive power are convex andconcave, respectively. By such arrangements, the image quality can beimproved effectively.

The first lens element provides a strong positive refractive power, andthe aperture stop is located close to the object side, so that the exitpupil of the optical lens assembly will be far away from the imageplane. Therefore, the light will be projected onto the photosensitiveassembly with a relatively small incident angle, this is the telecentricfeature of the image side, and this feature is very important to thephotosensitive power of the current solid photosensitive assembly, andcan improve the photosensitivity of the photosensitive assembly whilereducing the probability of the occurrence of shading. And at least oneinflection point is formed on the aspheric surface of the third lenselement to contribute to a better correction of the incident angle ofthe off axis light with respect to the photosensitive assembly. Inaddition, for a wide-angle optical system, it particularly needs tocorrect the distortion and the chromatic aberration of magnification,the correction method is to arrange the aperture stop at the balancepoint of the refractive power of the optical system. And the aperturestop of the optical system of the present invention is located betweenthe first lens element and the second lens element, the purpose of suchan arrangement is to balance the telecentric and wide field of view.Further, the position of the aperture stop can effectively reduce therefraction angle of the light with respect to the lens element, thusreducing the sensitivity of the optical system.

With the trend of miniaturization of the optical lens assembly and therequirement of increasing the field of view, the focal length of theoptical system is becoming shorter. Therefore, the radius of curvatureand the size of the lens elements must be reduced, and it is impossibleto make such glass lenses by the use of the conventional grind method.Plastic material is introduced to make lenses by injection molding,using a relatively low cost to produce high precision lenses. Thesurface of lens is aspheric for obtaining much more controlledvariables, so as to reduce the aberration and the number of the lenses,so that the length of the optical system can be reduced effectively.

In the optical system for taking image, an Abbe number of the secondlens element is V2, and it satisfies the relation:V2<40

The abovementioned relation can effectively correct the chromaticaberration caused by the system, and improve the resolution of theoptical system for taking image.

And it will be better if the Abbe number V2 of the second lens elementsatisfies the relation:V2<28

Further, it will be much better if the Abbe number V2 of the second lenselement satisfies the relation:V2<25

In the optical system for taking image, the Abbe number of the firstlens element is V1, the Abbe number of the third lens element is V3, andthey satisfy the relations:V1>50V3>50

The abovementioned relation can effectively correct the chromaticaberration caused by the system. And it will be better if the Abbenumber V1 of the first lens element satisfies the relation:V1>58

In the optical system for taking image, the refractive index of thesecond lens element is N2, and it satisfies the relation:N2<1.65

If the refractive index of the second lens element is higher than thehigh limit as stated above, it will be difficult to find an appropriateplastic optical material to match the optical system.

In the optical system for taking image, the focal length of the firstlens element is f1, the focal length of the optical system is f, andthey satisfies the relation:f/f1>0.95

The abovementioned relation can provide enough refractive power for theoptical system and can reduce the total length of the optical system.Further, it will be better if f/f1 satisfies the relation:f/f1>1.2

And it will be much better if f/f1 satisfies the relation:f/f1>1.25

In the optical system for taking image, the focal length of the secondlens element is f2, the focal length of the optical system is f, andthey satisfies the relation:|f/f2|>0.34|f/f2|<0.9

If the value of |f/f2| is smaller than the abovementioned low limit, itwill be difficult to correct the chromatic aberration caused by theoptical system, and if the value of |f/f2| is greater than theabovementioned high limit, the length of the optical system for takingimage will be too long, and this will be counter to the objective ofminiaturization of the optical system for taking image.

In the optical system for taking image, the focal length of the thirdlens element is f3, the focal length of the optical system is f, andthey satisfy the relation:f/f3<0.25

The third lens element serves as a correcting lens to balance andcorrect the various aberrations caused by the optical system. If thevalue of f/f3 is greater than the abovementioned high limit, the backfocal length of the optical system will be too short.

In the optical system for taking image, the radius of curvature of thefront surface of the first lens element is R1, the radius of curvatureof the rear surface of the first lens element is R2, and they satisfythe relation:0<R1/R2<0.5

If the value of R1/R2 is lower than the low limit as stated above, itwill be difficult to correct the Astigmatism caused by the opticalsystem for taking image. On the other hand, if the value of R1/R2 ishigher than the abovementioned high limit, it will be difficult tocorrect the spherical aberration caused by the optical system. And itwill better if the value of R1/R2 satisfies the relation:0.1<R1/R2<0.2

In the optical system for taking image, the tangential angle at theposition of the effective diameter of a rear surface of the third lenselement is ANG32, and it satisfies the relation:ANG32←30 deg.

The tangential angle at the position of the effective diameter of thesurface is defined as: The angle between the line perpendicular to theoptical axis and the tangential line at the position the surface. Whenthe tangential angle at the position of the effective diameter of thesurface is inclined toward the image side, the tangential angle ispositive, and when the tangential angle at the position of the effectivediameter of the surface is inclined toward the object side, thetangential angle is negative.

In the optical system for taking image, the height at the position ofthe effective diameter of the rear surface of the third lens element isSAG32, and it satisfies the relation:SAG32←0.2 mm

The height at the position of the effective diameter of the surface isdefined as: the distance between the position at the effective diameterand the position at the optical axis, which projects onto the opticalaxis. When the height at the position of the effective diameter of thesurface is inclined toward the image side, the height is positive, andwhen the height at the position of the effective diameter of the surfaceis inclined toward the object side, the height is negative. Theabove-mentioned relations can effectively reduce the incident angle ofthe light with respect to the photosensitive assembly while improvingthe correction of the off axis aberration.

In the optical system for taking image, the center thickness of thesecond lens element is CT, the edge thickness of the first lens elementis ET1, they satisfy the relations:CT2<0.4 mmET1<0.4 mm

The edge thickness is: the length projected on an optical axis by thedistance between the positions of the effective diameter of the frontand the rear surfaces of the lens. The abovementioned relations canreduce the height of the optical system while improving the imagequality.

In the optical system for taking image, the distance between the secondlens element and the third lens element is T23, and it satisfies therelation:T23>0.2 mm

The abovementioned relation can effectively improve the correction ofthe off axis aberration, and it will be better if T23 satisfies therelation:T23>0.38 mm

In the optical system for taking image, at least one inflection point isformed on the rear surface of the first lens element, so as toeffectively improve the image quality.

In the optical system for taking image, an object to be photographed isimaged on an electronic photosensitive assembly, a length of the opticalsystem is TL, an image height of the optical system is ImgH, and theysatisfy the relation:TL/ImgH<2.05

The abovementioned relation contributes to the miniaturization of theoptical system for taking image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an optical system for taking image in accordance with afirst embodiment of the present invention;

FIG. 2 shows the aberration curve of the first embodiment the presentinvention;

FIG. 3 shows an optical system for taking image in accordance with asecond embodiment of the present invention; and

FIG. 4 shows the aberration curve of the second embodiment the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, which shows an optical system for taking image inaccordance with a first embodiment of the present invention, and FIG. 2shows the aberration curve of the first embodiment of the presentinvention. The first embodiment of the present invention is an opticalsystem for taking image comprising three lens elements with refractivepower, and the optical system for taking image comprises: from theobject side to the image side:

A plastic first lens element 10 with positive refractive power has aconvex front surface 11 and a concave rear surface 12, the front surface11 and the rear surface 12 of the first lens element are aspheric, andan inflection point is formed on the rear surface 12.

A plastic second lens element 20 with negative refractive power has aconcave front surface 21 and a convex rear surface 22, and the frontsurface 21 and the rear surface 22 of the second lens element areaspheric.

A plastic third lens element 30 with positive refractive power has aconvex front surface 31 and a concave rear surface 32, the front surface31 and the rear surface 32 of the third lens element are aspheric, andthe third lens element 30 is formed with an inflection point.

An aperture stop 40 is located between the first lens element 10 and thesecond lens element 20 for controlling brightness of the optical system.

An IR cut filter 50 is located behind the third lens element 30 and hasno influence on the focal length of the optical system.

A sensor cover glass 60 is located behind the IR cut filter 50 and hasno influence on the focal length of the optical system.

An image plane 70 is located behind the sensor cover glass 60.

The equation of the curve of the aspheric surfaces is expressed asfollows:X(Y)=(Y ² /R)/(1+sqrt(1−(1+k)*(Y/R)²))+A ₄ *Y ⁴ +A ₆ *Y ⁶+ . . .

wherein:

X: represents the distance of the cross section of the lens;

Y: the height from the point on the curve of the aspheric surface to theoptical axis;

K: represents the conical coefficients;

A4, A6 . . . : the aspheric surface coefficients of the fourth and sixthorder.

In the optical system for taking image of the first embodiment, the Abbenumber of the first lens element is V1, the Abbe number of the secondlens element is V2, and the Abbe number of the third lens element is V3,wherein V1=60.3, V2=26.6, V3=55.8.

In the optical system for taking image of the first embodiment, therefractive index of the second lens element is N2, and N2=1.606.

In the optical system for taking image of the first embodiment, thefocal length of the first lens element is f1, the focal length of thesecond lens element is f2, the focal length of the third lens element isf3, the focal length of the optical system for taking image is f, andthey satisfy the relations: f/f1=1.26, |f/f2|=0.36, f/f3=0.15

In the optical system for taking image of the first embodiment, theradius of curvature of the front surface of the first lens element isR1, and the radius of curvature of the rear surface of the first lenselement is R2, they satisfy the relation: R1/R2=0.13.

In the optical system for taking image of the first embodiment, thetangential angle at the position of the effective diameter of a rearsurface of the third lens element is ANG32, and ANG32=−34.1 deg.

The tangential angle at the position of the effective diameter of thesurface is defined as: The angle between the line perpendicular to theoptical axis and the tangential line at the position the surface. Whenthe tangential angle at the position of the effective diameter of thesurface is inclined toward the image side, the tangential angle ispositive, and when the tangential angle at the position of the effectivediameter of the surface is inclined toward the object side, thetangential angle is negative.

In the optical system for taking image of the first embodiment, theheight at the position of the effective diameter of the rear surface ofthe third lens element is SAG32, and SAG32=−0.25 mm.

The height at the position of the effective diameter of the surface isdefined as: the distance between the position at the effective diameterand the position at the optical axis, which projects onto the opticalaxis. When the height at the position of the effective diameter of thesurface is inclined toward the image side, the height is positive, andwhen the height at the position of the effective diameter of the surfaceis inclined toward the object side, the height is negative.

In the optical system for taking image of the first embodiment, the edgethickness of the first lens element is ET1, the center thickness of thesecond lens element is CT2, the distance between the second lens elementand the third lens element is T23, and they satisfy the relations:ET1=0.400 mm, CT2=0.400 mm, T23=0.403 mm.

The edge thickness is: the length projected on an optical axis by thedistance between the positions of the effective diameter of the frontand the rear surfaces of the lens.

In the optical system for taking image of the first embodiment, thelength of the optical system is TL, the image height of the opticalsystem is ImgH, and they satisfy the relation: TL/ImgH=1.94.

The data of the structure of the first embodiment is shown in table 1,and the aspheric surface data is shown in table 2, wherein the units ofthe radius of curvature, the thickness and the focal length areexpressed in mm, and HFOV is half of the maximal field of view.

TABLE 1 (Embodiment 1) f(focal length) = 4.05 mm, Fno = 2.8, HFOV (halfof field of view) = 33.0 deg. Surface Curvature Thick- Abbe Focal #Radius ness Material Index # length 0 Object Plano Infinity 1 Lens 1 1.55698 0.655 Plastic 1.543 60.3 3.22 (ASP) 2 11.75710 0.047 (ASP) 3Ape. Plano 0.698 Stop 4 Lens 2 −0.73359 0.400 Plastic 1.606 26.6 −11.45(ASP) 5 −0.98900 0.403 (ASP) 6 Lens 3  3.98390 1.101 Plastic 1.53  55.827.45 (ASP) 7  4.95130 0.200 (ASP) 8 IR-filter Plano 0.300 Glass 1.51764.1 — 9 Plano 0.200 10  Cover Plano 0.500 Glass 1.517 64.1 — Glass 11 Plano 0.626 12  Image Plano

TABLE 2 Aspheric Coefficient Surface # 1 2 4 5 6 7  K =   1.37587E−01  9.62422E+01 −3.07134E+00 −6.13897E−01 −3.52240E+01 −2.74654E+01  A4 =−7.56636E−03 −7.5168E−02 −7.06220E−01   6.38934E−02 −1.95150E−02−3.90432E−02  A6 = −7.19498E−02 −1.03333E−01   7.46874E−01 −4.54511E−02  5.34151E−03   6.39075E−03  A8 =   1.16863E−01   3.02141E−01  2.97293E−01   4.36847E−01 −3.81336E−04 −1.43844E−03 A10 = −1.88319E−01−1.11220E+00 −8.00480E−01 −1.57047E−01 —   1.61965E−04 A12 =  4.00264E−02   1.10968E+00   9.33745E−02 −8.73112E−02 — −4.80071E−06A14 = — — −1.20299E−02   3.35883E−02 — —

Referring to FIG. 3, which shows an optical system for taking image inaccordance with a second embodiment of the present invention, and FIG. 4shows the aberration curve of the second embodiment of the presentinvention. The second embodiment of the present invention is an opticalsystem for taking image comprising three lens elements with refractivepower, and the optical system for taking image comprises: from theobject side to the image side:

A plastic first lens element 10 with positive refractive power has aconvex front surface 11 and a concave rear surface 12, and the frontsurface 11 and the rear surface 12 of the first lens element areaspheric.

A plastic second lens element 20 with negative refractive power has aconcave front surface 21 and a convex rear surface 22, and the frontsurface 21 and the rear surface 22 of the second lens element areaspheric.

A plastic third lens element 30 with positive refractive power has aconvex front surface 31 and a concave rear surface 32, the front surface31 and the rear surface 32 of the third lens element are aspheric, andthe third lens element 30 is formed with an inflection point.

An aperture stop 40 is located between the first lens element 10 and thesecond lens element 20 for controlling brightness of the optical system.

An IR cut filter 50 is located behind the third lens element 30 and hasno influence on the focal length of the optical system.

A sensor cover glass 60 is located behind the IR cut filter 50 and hasno influence on the focal length of the optical system.

An image plane 70 is located behind the sensor cover glass 60.

The equation of the curve of the aspheric surfaces of the secondembodiment is the same as that of the first embodiment.

In the optical system for taking image of the second embodiment, theAbbe number of the first lens element is V1, the Abbe number of thesecond lens element is V2, the Abbe number of the third lens element isV3, and V1=60.3, V2=23.4, V3=55.8.

In the optical system for taking image of the second embodiment, therefractive index of the second lens element is N2, and N2=1.632.

In the optical system for taking image of the second embodiment, thefocal length of the first lens element is f1, the focal length of thesecond lens element is f2, the focal length of the third lens element isf3, the focal length of the optical system for taking image is f, andthey satisfy the relations: f/f1=1.22, |f/f2|=0.44, f/f3=0.24.

In the optical system for taking image of the second embodiment, theradius of curvature of the front surface of the first lens element isR1, and the radius of curvature of the rear surface of the first lenselement is R2, they satisfy the relation: R1/R2=0.27.

In the optical system for taking image of the second embodiment, thetangential angle at the position of the effective diameter of a rearsurface of the third lens element is ANG32, and ANG32=−39.6 deg.

The definition of the tangential angle ANG32 of the second embodiment isthe same as that of the first embodiment.

In the optical system for taking image of the second embodiment, theheight at the position of the effective diameter of the rear surface ofthe third lens element is SAG32, and SAG32=−0.14 mm.

The definition of the height SAG32 of the second embodiment is the sameas that of the first embodiment.

In the optical system for taking image of the second embodiment, theedge thickness of the first lens element is ET1, the center thickness ofthe second lens element is CT2, the distance between the second lenselement and the third lens element is T23, and ET1=0.359 mm, CT2=0.350mm, T23=0.070 mm.

The definition of the edge thickness of the second embodiment is thesame as that of the first embodiment.

In the optical system for taking image of the second embodiment, thelength of the optical system is TL, the image height of the opticalsystem is Imgh, and they satisfy the relation: TL/ImgH=2.03.

The data of the structure of the second embodiment is shown in table 3,and the aspheric surface data is shown in table 4, wherein the units ofthe radius of curvature, the thickness and the focal length areexpressed in mm, and HFOV is half of the maximal field of view.

TABLE 3 (Embodiment 2) f(focal length) = 2.89 mm, Fno = 2.8, HFOV (halfof field of view) = 31.7 deg. Surface Curvature Thick- Abbe Focal #Radius ness Material Index # length 0 Object Plano Infinity 1 Lens 1 1.01836 0.602 Plastic 1.543 60.3 2.36 ASP 2  3.82400 0.053 (ASP) 3 Ape.Infinity 0.528 Stop 4 Lens 2 −0.62407 0.350 Plastic 1.632 23.4 −6.56(ASP) 5 −0.89356 0.070 (ASP) 6 Lens 3  2.02762 0.697 Plastic 1.53 55.812.16 (ASP) 7  2.59896 0.150 (ASP) 8 IR-filter Plano 0.300 Glass 1.51764.1 — 9 Plano 0.050 10  Cover Plano 0.550 Glass 1.517 64.1 — Glass 11 Plano 0.301 12  Image Plano

TABLE 4 Aspheric Coefficient Surface # 1 2 4 5 6 7  K = −4.48235E−01−3.31506E+01 −1.12702E+00 −1.81125E−01 −2.90359E+01 −6.90682E+00  A4 =  3.65670E−02   5.12100E−02 −1.36123E−01 −8.93404E−02 −1.24396E−01−1.68098E−01  A6 =   9.28051E−02 −6.00915E−01 −2.91997E+00   5.88791E−01  1.34962E−01   6.08137E−02  A8 = −1.73164E−01   1.02124E+00  1.79892E+01   1.64984E−01 −7.56360E−02 −2.35830E−02 A10 = — —−6.29711E+01 —   2.09185E−02   6.08246E−03 A12 = — —   6.53559E+01 —−2.34988E−03 −1.07127E−03

TABLE 5 Embodiment 1 Embodiment 2 f 4.05 2.89 Fno 2.8 2.8 HFOV 33.0 31.7V1 60.3 60.3 V2 26.6 23.4 V3 55.8 55.8 N2 1.606 1.632 f/f1 1.26 1.22|f/f2| 0.36 0.44 f/f3 0.15 0.24 R1/R2 0.13 0.27 ANG32 −34.1 −39.6 SAG32−0.25 −0.14 CT2 0.400 0.350 ET1 0.400 0.359 T23 0.403 0.070 TL/ImgH 1.942.03

It is to be noted that the tables 1-4 show different data of thedifferent embodiments, however, the data of the different embodiments isobtained from experiments. Therefore, any product of the same structureis contemplated to be within the scope of the present invention even ifit uses different data. Table 5 is the data of the respectiveembodiments resulted from the equations.

While we have shown and described various embodiments in accordance withthe present invention, it is clear to those skilled in the art thatfurther embodiments may be made without departing from the scope of thepresent invention.

1. An optical system for taking image comprising three lens elementswith refractive power, from the object side to the image side: a firstlens element with positive refractive power having a convex frontsurface and a concave rear surface, the front surface of the first lensbeing aspheric; a plastic second lens element with negative refractivepower having a concave front surface and a convex rear surface, thefront surface and the rear surface of the second lens being aspheric; aplastic third lens element with positive refractive power having aconvex front surface and a concave rear surface, the front surface andthe rear surface of the third lens being aspheric; and an aperture stoplocated between the first lens element and the second lens element forcontrolling brightness of the optical system; wherein a focal length ofthe first lens element is f1, a focal length of the second lens elementis f2, a focal length of the optical system is f, and they satisfy therelations: f/f1>0.95, |f/f2|>0.34.
 2. The optical system for takingimage as claimed in claim 1, wherein the first lens element is made ofplastic material, the rear surface of the first lens element isaspheric, and the third lens element is formed with at least oneinflection point.
 3. The optical system for taking image as claimed inclaim 2, wherein an Abbe number of the second lens element is V2, and itsatisfies the relation: V2<40.
 4. The optical system for taking image asclaimed in claim 3, wherein an Abbe number of the second lens element isV2, and it satisfies the relation: V2<28.
 5. The optical system fortaking image as claimed in claim 4, wherein an Abbe number of the secondlens element is V2, and it satisfies the relation: V2<25.
 6. The opticalsystem for taking image as claimed in claim 5, wherein a refractiveindex of the second lens element is N2, and it satisfies the relation:N2<1.65.
 7. The optical system for taking image as claimed in claim 4,wherein a radius of curvature of the front surface of the first lenselement is R1, and a radius of curvature of the rear surface of thefirst lens element is R2, they satisfy the relation: 0<R1/R2<0.5.
 8. Theoptical system for taking image as claimed in claim 7, wherein a radiusof curvature of the front surface of the first lens element is R1, and aradius of curvature of the rear surface of the first lens element is R2,they satisfy the relation: 0<R1/R2<0.2.
 9. The optical system for takingimage as claimed in claim 4, wherein a tangential angle ANG32 at aposition of an effective diameter of a rear surface of the third lenselement satisfies the relation: ANG32←30 deg.
 10. The optical system fortaking image as claimed in claim 9, wherein a height SAG32 at theposition of the effective diameter of a rear surface of the third lenselement satisfies the relation: SAG32←0.2 mm.
 11. The optical system fortaking image as claimed in claim 4, wherein a center thickness of thesecond lens element is CT2, and it satisfies the relation: CT2<0.4 mm.12. The optical system for taking image as claimed in claim 11, whereinan edge thickness of the first lens element is ET1, and it satisfies therelation: ET1<0.4 mm.
 13. The optical system for taking image as claimedin claim 12, wherein a distance between the second lens element and thethird lens element is T23, and it satisfies the relation: T23>0.2 mm.14. The optical system for taking image as claimed in claim 13, whereina distance between the second lens element and the third lens element isT23, and it satisfies the relation: T23>0.38 mm.
 15. The optical systemfor taking image as claimed in claim 2, wherein the focal length of thefirst lens element is f1, the focal length of the optical system is f,and they satisfy the relation: f/f1>1.2.
 16. The optical system fortaking image as claimed in claim 15, wherein the focal length of thefirst lens element is f1, the focal length of the optical system is f,and they satisfy the relation: f/f1>1.25.
 17. The optical system fortaking image as claimed in claim 16, wherein an Abbe number of thesecond lens element is V2, and it satisfies the relation: V2<28.
 18. Theoptical system for taking image as claimed in claim 7, wherein a focallength of the second lens element is f2, a focal length of the opticalsystem is f, and they satisfy the relation: |f/f2|<0.9.
 19. The opticalsystem for taking image as claimed in claim 7, wherein a focal length ofthe third lens element is f3, a focal length of the optical system is f,and they satisfy the relation: |f/f3|<0.25.
 20. The optical system fortaking image as claimed in claim 7, wherein an Abbe number of the firstlens element is V1, an Abbe number of the third lens element is V3, andthey satisfy the relations: V1>50, V3>50.
 21. The optical system fortaking image as claimed in claim 20, wherein the Abbe number of thefirst lens element is V1, and it satisfies the relation: V1>58.
 22. Theoptical system for taking image as claimed in claim 7, wherein at leastone inflection point is formed on the rear surface of the first lenselement.
 23. The optical system for taking image as claimed in claim 4,wherein an object to be photographed is imaged on an electronicphotosensitive assembly, a length of the optical system is TL, an imageheight of the optical system is ImgH, and they satisfy the relation:TL/ImgH<2.05.